The present invention relates to a rechargeable battery and a method of manufacturing the same, and more particularly, to the reduction of internal resistance in a rechargeable battery having a configuration in which power generation elements respectively accommodated in battery jars connected to one another through partition walls, are connected one another.
An example of a rechargeable battery in the prior art will now be described with reference to FIG. 1. A rechargeable battery 100 includes a thin rectangular battery jar 3 formed by connecting a plurality of bar-shaped battery jars 4. Each battery jar 4 has a narrow side and a wide side. The wall on the narrow side is shared by an adjacent battery jar 4 and functions as a partition wall 5. The battery jars 4 have upper open ends closed by the same integral lid 6. Connection holes 7 are formed at the upper parts of the outer narrow walls of the two battery jars 4 located at the two ends of the connected battery jars 4 and the partition walls 5 between the adjacent battery jars 4. A power generation element 11, which includes an electrode plate assembly 8 of a cathode plate and an anode plate stacked together with a separator arranged in between, and collector plates 10a and 10b, which are connected to the two ends of the electrode plate assembly 8, are accommodated in each battery jar 4 together with an electrolysis solution to form a cell 2. The electrode plate assembly 8 includes leads 9a and 9b respectively provided for the cathode plate and the anode plate and extending from opposite sides. The collector plates 10a and 10b are respectively connected to the sides of the leads 9a and 9b. 
A connection projection 12, which is fitted into the connection hole 7, projects from the upper part of each of the collector plates 10a and 10b. Spot welding is performed to connect the connection projections 12 of the collector plates 10a and 10b between the two adjacent battery jars 4. The spot welding is performed in the upper opening of the battery jar 4. Cathode or anode connection terminals 13 are attached to the connection holes 7 in the outer narrow walls of the two battery jars 4 located at the two ends of the connected battery jars 4. The connection terminals 13 are spot welded and connected to the connection projections 12 of the associated collector plates 10a and 10b. In this manner, the plurality of battery cells 2 embedded in the rectangular battery jar 3 are connected in series to form a rechargeable battery having connection terminals on its two both ends (refer to, for example, Japanese Laid-Open Patent Publication No. 2001-93503). In FIG. 1, a communication passage 24 functions to equalize the inner pressure of the connected battery jars 4. A hole 25 is formed to receive a temperature sensor. A safety valve (not shown) is provided to release pressure when the inner pressure of the battery jars 4 become higher than or equal to a certain pressure.
A conductive path extending from the cathode plate and the anode plate to the corresponding leads 9a and 9b and collector plates 10a and 10b is short in the prior art rechargeable battery 100 shown in FIG. 1. However, the collector plates 10a and 10b are connected to the distal ends of the connection projections 12 at the upper portions of the collector plates 10a and 10b at one location as shown by arrows in FIG. 2. Thus, the connection path detoured in such a manner lengthens the conductive path. Further, the collector plates 10a and 10b are connected to each other at only one location. Therefore, the internal resistance becomes high in the rechargeable battery 100 of the prior art. Additionally, the detoured connection path results in uneven current distribution between portions close to and portions distant from the connected portion of the collector plates 10a and 10b as shown by the white arrows. Thus, the entire electrode plate assembly 8 cannot generate power evenly thereby causing a large loss in the rechargeable battery 100 of the prior art.
To solve such a problem, a further rechargeable battery 200 has been proposed in the prior art (refer to, for example, Japanese Laid-Open Patent Publication No. 2003-282043). As shown in FIGS. 3 and 4, in addition to the upper parts of the collector plates 10a and 10b being connected through the connection hole 7 formed at the upper part of the partition wall 5, the rechargeable battery 200 includes an opening 31 formed at the lower part of at least one side wall 14 of a rectangular battery jar 3. The openings face the interiors of the battery jars 4 on opposite sides of the partition wall 5. Two electrically conductive connection plates 33 are arranged on opposite sides of the partition wall 5. Each connection plate 33 includes a connection projection 34 and a connection piece 35. The connection projections 34 of the connection plates 33 are connected to each other through a connection hole 32, which is formed at the lower part of each partition wall 5, by a welded portion 36. The connection pieces 35 face toward the corresponding openings 31. The collector plates 10a and 10b contact the inner surface of the connection piece 35 with the power generation element 11 accommodated in the corresponding battery jar 4. In a state in which a welding electrode 40 is in contact with the connection pieces 35 located on each side of the partition wall 5, welding current flows to form a welded portion 37 connecting the collector plates 10a and 10b and the connection piece 35. Then, the opening 31 is sealed with a sealing plate 38 so as to isolate each battery jar 4 from one another. A seal 39 is arranged between the connection hole 32 and the connection projections 34. Joining portions 10c are defined on the collector plates 10a and 10b at positions joined with the leads 9a and 9b. 
In the prior art rechargeable battery 200 shown in FIGS. 3 and 4, the collector plates 10a and 10b are connected at both the upper part and the lower part. Thus, the internal resistance is reduced to a certain extent, and the current distribution flowing through the electrode plate assembly is equalized to a certain extent. However, the collector plates 10a and 10b are directly connected to each other through welding at the upper connection portion, whereas the collector plates 10a and 10b are connected by the electrically conductive connection plates 33 at the lower connection portion. Therefore, in the prior art rechargeable battery 200, the welded portion 37 forms a connection resistance between the collector plates 10a and 10b and the connection piece 35. The electrically conductive connection plate 33 also forms an electrically conductive resistance. Further, the welded portion 36 connecting the two connection projections 34 forms a connection resistance. The sum of these resistances causes the connection resistance at the lower part to be relatively larger than the connection resistance at the upper part. As a result, the reduction of the internal resistance and the distribution of equalized current flowing through the electrode plate assembly cannot be sufficiently achieved.
When describing this with specific numerical values, the resistance for each cell 2 at the upper connection part where the connection projections 12 are welded is 0.03 mΩ. Comparatively, the connection resistance between the collector plates 10a and 10b and the connection piece 35 at the lower connection part is 0.05 mΩ. The electrically conductive resistance of the electrically conductive connection plate 33 is 0.05 mΩ. The resistance for each cell 2 at the welded portion 36 where the connection projections 34 are welded is 0.03 mΩ. This amounts to a total of 0.13 mΩ and indicates that the resistance at the lower connection part is significantly greater than that at the upper connection part. Even if the width of the electrically conductive connection plates 33 were increased and a plurality of (two) connection projections 34 were used, the resistance would still be 0.08 mΩ. Furthermore, even if the connection pieces 35 were to be welded at two sides of the collector plates 10a and 10b, the resistance would still be 0.68 mΩ. Thus, the resistance of the lower connection part would still be larger than that at the upper connection part.